1. Field of the Invention
This invention relates to corrosion inhibiting compositions and more particularly to corrosion inhibitors for use in calcium-free brines which may be used in formations having high carbonate and high sulfate concentrations.
2. Description of the Prior Art
Aqueous solutions of alkali and alkaline earth metal and zinc halides, and mixtures thereof, are used as drilling, completion, packer and perforating media in oil and gas wells. Depending on the formation pressures encountered in the well, fluids with densities between 8.5 and 20.0 lb/gal are designed to provide a hydrostatic head to balance the formation pressure. The advent of the so-called solids-free brines in recent years has greatly improved the completion and workover activities by eliminating formation plugging and solid settling problems previously encountered when water base and oil base drilling fluids were used.
There are oil field uses for clear fluids in which subterranean carbonate and sulfate concentrations cause calcium carbonate and calcium sulfate precipitation from the clear fluids which contain significant calcium ion concentration. In some of these instances, sodium bromide fluid has been used in place of calcium ion-containing fluids in relatively shallow holes where high formation pressures are not encountered.
It has been reported by Shaughnessy, et al., "Workover Fluids for Prudhoe Bay", February-July 1977, that the mixing of calcium chloride workover fluid with formation brine can lead to the precipitation of calcium carbonate within the reservoir rock at Prudhoe Bay (P=5000 psi; T=220.degree. F.). To minimize formation damage, a nondamaging fluid, sodium bromide, was used. More recently, carbonate and sulfate containing wells have been found requiring fluid densities well above the sodium bromide range, i.e., 14-20 lb/gal.
U.S. patent application Ser. No. 892,155, filed Jul. 30, 1986, now abandoned, discloses new solids-free brine comprising aqueous solutions of zinc bromide, sodium bromide, potassium bromide, and lithium bromide. According to this disclosure, base fluids of zinc bromide/sodium bromide (density=17.5 lb/gal), zinc bromide/potassium bromide (density=18.0 lb/gal), and zinc bromide/lithium bromide (density=18.0 lb/gal) were prepared by mixing zinc bromide solutions (60-70 wt. %) with the respective monovalent salts (sodium brmide, potassium bromide, or lithium bromide). Different calcium-free fluids covering the density range from 11.5 to 18.0 lb/gal were then formulated by mixing the base fluids with 12.5 lb/gal sodium bromide, 11.3 lb/gal potassium bromide, and 13.4 lb/gal lithium bromide solutions.
Considering the solubility products of calcium carbonate (3.8.times.10.sup.-9 at 25.degree. C.), zinc carbonate (2.1.times.10.sup.-11 at 25.degree. C.), and calcium sulfate (2.4.times.10.sup.-5 at 25.degree. C.), one would expect that zinc carbonate, calcium carbonate, and calcium sulfate should precipitate when zinc bromide/calcium bromide fluids are mixed with formation brine with high carbonate and sulfate ion concentration. The novelty of the calcium-free fluids is that the absence of calcium ion by the substitution of either sodium bromide, potassium bromide or lithium bromide for calcium bromide, lowers the total concentration of divalent metal ions in calcium-free fluids compared with that of standard zinc bromide/calcium bromide fluids such that, when either 17.5 lb/gal zinc bromide/sodium bromide or 18.0 lb/gal zinc bromide/potassium bromide or zinc bromide/lithium bromide are mixed with a 2/8 ratio of formation brine having high carbonate and sulfate ion concentrations, no precipitate will form.
Although solids-free fluids have improved the drilling and completion efficiency, these fluids can be corrosive under operating conditions in drilling, completion, packer and perforating operations. The extent of corrosion is increased at higher temperatures encountered in deeper wells. Any fluid containing zinc halides is more corrosive than those fluids formulated with alkali and alkaline earth metal halides. Therefore it is necessary to use an appropriate corrosion inhibitor when zinc ion is present in the fluid.
In order to minimize the corrosion of iron and steel casing, tubing and the equipment exposed to clear brines, special corrosion additives are combined with the brines before they are used. Corrosion inhibitors such as film-forming amine-based inhibitors are used for calcium-based fluids, and are described in U.S. Pat. No. 4,292,183. According to British patent No. 2,027,686 (Feb. 27, 1983) and German Offen. No. 3,316,677 Al (Dec. 8, 1983), corrosion inhibitors such as thiocyanate, thioglycolate or thiourea compounds, alone, or in combination with a quaternary pyridinium, quinolium, or isoquinolinium salts may be used in conventional calcium-ion containing concentrated salt solutions, such as zinc bromide, calcium chloride solutions. However, calcium free solutions are significantly more corrosive than zinc bromide/calcium bromide fluids, and conventional inhibitors are not expected to perform satisfactorily in calcium free fluids.
Accordingly, the objective of the present invention is to design corrosion inhibitor systems which can control the corrosion of iron and steel, casing, tubing, and other equipment exposed to high temperature and calcium-free fluids as completion, packer and perforating media. The corrosion inhibitors must be sufficiently soluble in the calcium-free fluids so that they can be formulated in the base fluids.